ABSTRACT In the most prevalent muscular dystrophy, Duchenne Muscular Dystrophy (DMD), repeated muscle degeneration and regeneration leads to muscle satellite cell (SC) dysfunction and/or exhaustion and there is no cure. In the previous funding period, we have identified novel differentiation and enrichment strategies to generate the most engraftable cells to date from human induced pluripotent stem cells (hiPSCs), as well as a CRISPR correction strategy to restore dystrophin applicable to 60% of patients. The next phase of this work is now focused on improving our understanding of the functional status of skeletal muscle progenitors (SMPCs) derived from wt, DMD and CRISPR corrected lines. This work will improve our understanding of the molecular and differences between human PAX7+ stem cells and progenitor cells across human fetal development through adulthood, and inform our ability to generate the most regenerative cells from hiPSCs in this funding period. In Aim 1, we will define human SMPCs and SCs arising in development and from hiPSCs and identify functional differences between progenitor and SC states across human development and in vitro derived cells using single cell sequencing and evaluation of candidate pathways different between SMPC and SC states. In Aim 2, we will evaluate the role of the host microenvironment including endogenous PAX7 cells on stem cell engraftment and ability to transition to SCs and reside in the SC niche. In Aim 3, we will utilize DMD and isogenic CRISPR/Cas9 SMPCs/SCs to evaluate specification, cell biology and functional potential of DMD and CRISPR corrected cells in diseased mdx-NSG and mdx-D2-NSG microenvironments. This will improve our understanding of differences and transitions between human muscle progenitor and stem cell states and will improve our ability to generate cells capable of repopulating the niche in long term studies.